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NAME
fenv.h - floating-point environment
SYNOPSIS
#include <fenv.h>
DESCRIPTION
The <fenv.h> header shall define the following data types through typedef:
fenv_t Represents the entire floating-point environment. The floating-point environment
refers collectively to any floating-point status flags and control modes supported
by the implementation.
fexcept_t
Represents the floating-point status flags collectively, including any status the
implementation associates with the flags. A floating-point status flag is a system
variable whose value is set (but never cleared) when a floating-point exception is
raised, which occurs as a side effect of exceptional floating-point arithmetic to
provide auxiliary information. A floating-point control mode is a system variable
whose value may be set by the user to affect the subsequent behavior of floating-
point arithmetic.
The <fenv.h> header shall define the following constants if and only if the implementation
supports the floating-point exception by means of the floating-point functions
feclearexcept(), fegetexceptflag(), feraiseexcept(), fesetexceptflag(), and
fetestexcept(). Each expands to an integer constant expression with values such that
bitwise-inclusive ORs of all combinations of the constants result in distinct values.
FE_DIVBYZERO
FE_INEXACT
FE_INVALID
FE_OVERFLOW
FE_UNDERFLOW
The <fenv.h> header shall define the following constant, which is simply the bitwise-
inclusive OR of all floating-point exception constants defined above:
FE_ALL_EXCEPT
The <fenv.h> header shall define the following constants if and only if the implementation
supports getting and setting the represented rounding direction by means of the
fegetround() and fesetround() functions. Each expands to an integer constant expression
whose values are distinct non-negative vales.
FE_DOWNWARD
FE_TONEAREST
FE_TOWARDZERO
FE_UPWARD
The <fenv.h> header shall define the following constant, which represents the default
floating-point environment (that is, the one installed at program startup) and has type
pointer to const-qualified fenv_t. It can be used as an argument to the functions within
the <fenv.h> header that manage the floating-point environment.
FE_DFL_ENV
The following shall be declared as functions and may also be defined as macros. Function
prototypes shall be provided.
int feclearexcept(int);
int fegetexceptflag(fexcept_t *, int);
int feraiseexcept(int);
int fesetexceptflag(const fexcept_t *, int);
int fetestexcept(int);
int fegetround(void);
int fesetround(int);
int fegetenv(fenv_t *);
int feholdexcept(fenv_t *);
int fesetenv(const fenv_t *);
int feupdateenv(const fenv_t *);
The FENV_ACCESS pragma provides a means to inform the implementation when an application
might access the floating-point environment to test floating-point status flags or run
under non-default floating-point control modes. The pragma shall occur either outside
external declarations or preceding all explicit declarations and statements inside a
compound statement. When outside external declarations, the pragma takes effect from its
occurrence until another FENV_ACCESS pragma is encountered, or until the end of the
translation unit. When inside a compound statement, the pragma takes effect from its
occurrence until another FENV_ACCESS pragma is encountered (including within a nested
compound statement), or until the end of the compound statement; at the end of a compound
statement the state for the pragma is restored to its condition just before the compound
statement. If this pragma is used in any other context, the behavior is undefined. If part
of an application tests floating-point status flags, sets floating-point control modes, or
runs under non-default mode settings, but was translated with the state for the
FENV_ACCESS pragma off, the behavior is undefined. The default state (on or off) for the
pragma is implementation-defined. (When execution passes from a part of the application
translated with FENV_ACCESS off to a part translated with FENV_ACCESS on, the state of the
floating-point status flags is unspecified and the floating-point control modes have their
default settings.)
The following sections are informative.
APPLICATION USAGE
This header is designed to support the floating-point exception status flags and directed-
rounding control modes required by the IEC 60559:1989 standard, and other similar
floating-point state information. Also it is designed to facilitate code portability
among all systems.
Certain application programming conventions support the intended model of use for the
floating-point environment:
* A function call does not alter its caller's floating-point control modes, clear its
caller's floating-point status flags, nor depend on the state of its caller's floating-
point status flags unless the function is so documented.
* A function call is assumed to require default floating-point control modes, unless its
documentation promises otherwise.
* A function call is assumed to have the potential for raising floating-point exceptions,
unless its documentation promises otherwise.
With these conventions, an application can safely assume default floating-point control
modes (or be unaware of them). The responsibilities associated with accessing the
floating-point environment fall on the application that does so explicitly.
Even though the rounding direction macros may expand to constants corresponding to the
values of FLT_ROUNDS, they are not required to do so.
For example:
#include <fenv.h>
void f(double x)
{
#pragma STDC FENV_ACCESS ON
void g(double);
void h(double);
/* ... */
g(x + 1);
h(x + 1);
/* ... */
}
If the function g() might depend on status flags set as a side effect of the first x+1, or
if the second x+1 might depend on control modes set as a side effect of the call to
function g(), then the application shall contain an appropriately placed invocation as
follows:
#pragma STDC FENV_ACCESS ON
RATIONALE
The fexcept_t Type
fexcept_t does not have to be an integer type. Its values must be obtained by a call to
fegetexceptflag(), and cannot be created by logical operations from the exception macros.
An implementation might simply implement fexcept_t as an int and use the representations
reflected by the exception macros, but is not required to; other representations might
contain extra information about the exceptions. fexcept_t might be a struct with a member
for each exception (that might hold the address of the first or last floating-point
instruction that caused that exception). The ISO/IEC 9899:1999 standard makes no claims
about the internals of an fexcept_t, and so the user cannot inspect it.
Exception and Rounding Macros
Macros corresponding to unsupported modes and rounding directions are not defined by the
implementation and must not be defined by the application. An application might use #ifdef
to test for this.
FUTURE DIRECTIONS
None.
SEE ALSO
The System Interfaces volume of IEEE Std 1003.1-2001, feclearexcept(), fegetenv(),
fegetexceptflag(), fegetround(), feholdexcept(), feraiseexcept(), fesetenv(),
fesetexceptflag(), fesetround(), fetestexcept(), feupdateenv()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .